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Unique properties of p block elements and their compounds. Elias lectures 2014 CYL 503 IITD. World chemicals turnover in 2010 was valued at 2353 billion Euros with China being the major producer of most of the bulk chemicals.

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World chemicals turnover in 2010 was valued at 2353 billion Euros with China being the major producer of most of the bulk chemicals.

Among industrially important inorganic chemicals and chemical compositions, those based on the p block elements constitute a major component if one keeps the organic carbon based compounds aside.

Among the top fifteen bulk inorganic chemicals/elements produced by chemical industries across the world, ten belong to the p block.

materials for infrastructure clay bricks (200 billion bricks per year), cement (3600 mmt)

glass (56 mmt)

Natural zeolites(3 mmt).

Three major organic chemicals produced in the word in 2013

ethylene (156 mmt)

propylene (80 mmt)

methanol (65 mmt)

1million metric ton (mmt) =1 000 000 000 kilograms (10 crore kg!)


Common inorganic fertilizers

The multibillion dollar chemical equation!

Ca5(PO4)3X + 5 H2SO4 + 10 H2O → 3 H3PO4 + 5 CaSO4·2 H2O + HX

X=F (fluorapatite) OH hydroxyapatite)


Solar energy materials

The complete spectrum of sunlight, from infrared to ultraviolet, covers a bandgap range from approximately 0.5 eV to about 2.9 eV (red light 1.7 eV and blue light 2.7 eV).


Nuclear materials based on the p block

Lead can effectively attenuate certain kinds of radiation because of its relatively high density (11.3) and high atomic number. It is quite effective at stopping gamma rays, and X-rays. Lead’s high density is caused by the combination of its high atomic mass and the relatively small size of its bond lengths and atomic radius. Lead is ineffective in stopping beta particles because they can produce secondary radiation when passing through elements with a high atomic number and density (Bremsstrahlung). Instead, plastic can be used to form an efficient barrier for dealing with high-energy beta radiation before the radiation reaches the lead barrier.


1935 H J Taylor

Aerial view of Chernobyl reactor


Transparent conducting oxides (TCOs)

Transparent conducting oxides (TCOs) are electrical conductive materials with comparably low absorption of electromagnetic radiation within the visible region of the spectrum. Indium tin oxide is one of the most widely used transparent conducting oxide because of its unique properties such as good electrical conductivity (~104 S/cm), very high optical transparency (75-90%), as well as the ease with which it can be deposited as a thin film. Indium tin oxide is a mixture of In2O3 (90%) and SnO2 (10%). The conductive and highly transparent surface created by ITO when applied as a coating to glass or other transparent surfaces, reflect infrared rays while allowing visible and UV light to pass. ITO which has a stability upto 150 C is omnipresent in modern touch screen technology starting from automatic teller machines, touch screen mobile phones and tablet PCs. It is also used in flat panel LCD, OLED plasma and electro-chromatic displays, solar panels and even on the energy efficient windows of modern aircrafts for easy deicing.


Inorganic polymers

Among the three, the one which has the highest market in terms of quantity are the silicones. The global market for silicones in terms of revenues was estimated to be worth $13,000 million in 2011 and is expected to reach $19,000 million by 2017. The silicone market is dominated by elastomers and fluids, together accounting for over 80% of the overall market. Silicones possess superior properties and characteristics which are extremely useful in the automotive, construction, medical and personal care and, electrical and electronics end-user industries


Industrial success of Silicones : PDMS

Hydrophobicity Oxygen permeability

Wide temp range Low glass transition temp

Antifrothing Non toxicity

First human foot print on moon

Moon temperature

(-153 to +107 °C)


Light Emitting Diodes

The main P-type dopant used in the manufacture of Light Emitting Diodes is Gallium and that the main N-type dopant used is Arsenic giving the resulting compound of Gallium Arsenide (GaAs) .The problem with using Gallium Arsenide on its own as the semiconductor compound is that it radiates large amounts of low brightness infra-red radiation (850nm-940nm) from its junction when a forward current is flowing through it. But by adding Phosphorus, as a third dopant the overall wavelength of the emitted radiation is reduced to below 680nm giving visible red light to the human eye.


As reagents and catalysts

Lewis acid catalysts based on main group metals such as aluminum, boron, silicon,  tin and antimony (BF3, SnCl4, SiF4, SbF5 and AlCl3 ) are used extensively in reactions such as Friedel-Crafts and the aldol reaction, and various pericyclic processes that proceed slowly at room temperature, such as the Diels-Alder reaction, Claisen rearrangement  and acetal formation.

The unusual catalytic activity shown by aluminosilicates such as zeolites and clays (e.g. montmorillonite) are unique to the p block elements. The open framework stable structures such as zeolites and ALPO can acts as dehydration and shape selective catalysts. Also of importance are methylating agents such as AlMe3 and its partially hydrolyzed product methyl alumoxane (MAO) which find application as cocatalysts in olefin polymerization reactions


Si 50% - Al 50%

Cations Na, Ca


3-D straight channels

Linde A

Ring 8

Si 93 %



1-D channels


Ring 10

Si x% - Al y%

Cations Na, Ca


3-D entangled channels


Ring 12


Periodicity of properties of p block elements


Li= -3.0

Inert pair efffect


Valence shell electron pair repulsion theory (VSEPR)

The rules are based on the Points on a sphere modelas it envisages the arrangement of electron pairs as points on a spherical core. The rules have been quite successful in predicting the shape of compounds whose central atom is a p block element and not very useful in the case of compounds of transition metals .

Ronald Gillespie

Mc Master Univ., Canada

The "AXE method" of electron counting

A represents the central atom and always has an implied subscript one. Xrepresents the number of ligands (atoms bonded to A).

E represents the number of lone pairs surrounding the central atom. 

The electrons are arranged in pairs that are either bonding (shared) pairs [X] or non bonding (unshared) pairs [E]. In the usual  -  treatment this usually means ignoring the  bonds temporarily since they will follow the  bonds and therefore the two bonding pairs of a double bond or three bonding pairs of a triple bond are all primarily considered as a single point on the sphere.

e.g. H2O AX2E2 2 bonding pairs and 2 lone pairs on Oxygen as central atom

O=PCl3 AX4 4 bonding pairs  bond ignored with phosphorus as central atom


The rules

The pairs of electrons in the valence shell adopt that arrangement which maximizes their distances apart. This in other words means that the bonding and non bonding electron pairs are arranged as far apart from each other as possible on the surface of a sphere with the atom A at the centre of the sphere.

Rules which take care of the deviations observed from the ideal geometry

Rule 1: A non bonding lone pair [E] occupies more space on the surface of the central atom than a bonding pair [X]. In other words, lone pair- lone pair repulsion is greater than lone pair- bond pair repulsion which is greater than bond pair-bond pair repulsion. Lone pairs choose the largest site, e.g., equatorial in trigonalbipyramid and axial in pentagonal bipyramid. If all sites are equal, lone pairs will be better placed trans to each other.


Rule 2:The strengths of the repulsions between bonds of different multiplicity decreases in the order: triple–double > double– double > double–single > single–single.

Rule 3: Bonding pairs to electronegative substituents occupy less space than those to more electropositive substituents. Therefore angles between the single bonds, decrease with increasing electronegativity of the substituent.

This decrease in domain size with increasing electronegativity of the substituent happens as more of the bonding electron density is shifted to the electronegative substituent. In other words, the repulsion between single bonds decreases with increasing electronegativity of the substituent and/or decreasing electronegativity of the central atom.


Shapes of compounds having odd number of electrons

The odd electron also has an influence on the molecular geometry similar to a normal lone pair but of relatively lesser strength. As a result, the geometry will be midway between the molecule with a full electron pair and the molecule with one less electron pair on the central atom.

Nitrogen dioxide (NO2) (AX2E0.5) bent shape with an ONO angle 134.

NO2 which is AX2E is also bent but has an ONO angle approximately of 120

whileNO2+ (AX2) is linear .

Similarly ClO2 (AX2E1.5) with a lone pair and an unpaired electron has OClO angle of 117.6

while ClO2with two lone pairs(AX2E2) has a OClO angle of 111.

Exceptions to the VSEPR rules

1. Metal complexes and organometallic compounds based on transition elements

2. Many triatomic alkaline earth metal dihalides, although expected to be linear have been found to be bent (approximate X-M-X angles: CaF2, 145°; SrF2, 120°; BaF2, 108°; SrCl2, 130°; BaCl2, 115°; BaBr2, 115° and  BaI2, 105°). Gillespie has proposed that the reason for the same is due to loss of spherical symmetry of the inner shell of the metal atom due to polarising effect of the relatively electronegative halide substituent’s.

Li2O also is linear although it is expected to be bent and this has been ascribed to the bonding being more ionic resulting in repulsion between the lithium atoms.


3. The silyl ether O(SiH3)2 has a relatively larger Si-O-Si angle (144.1°), while similar bond angles of Cl2O (110.9°) and (CH3)2O (111.7°) are in the expected range.

Gillespie\'s rationalization for this observation is that the localization of the lone pairs, and therefore their ability to repel other electron pairs, is greatest when the ligand has an electronegativity similar to or greater than, that of the central atom. When the central atom is more electronegative, as in O(SiH3)2, the lone pairs are less well-localised and have therefore a weaker repulsive effect.

4. Some molecules in which the central atom is from periods 3 and 4 and in which the ligands are less electronegative, do not have sufficient space in their valence shell to accommodate six bonding domains and a large lone pair domain. In such molecules the lone pair is squeezed into a spherical domain surrounding the core and inside the bonding domains (stereochemically inactive s orbital) which therefore have an octahedral arrangement. Thus some AX6E molecule such as SeCl62- and TeBr62- have a regular octahedral shape but with longer than normal bonds.


Some new developments

The electron pair domain model

proposed by Gillespie and Harigittai in 1991 followed by Gillespie and Robinson in 1996. This model was based not on pairs of electrons but on a region in space where there is more probability of finding an electron pair (bonding or lone pair).

An electron pair domain is a region in space where there is an enhanced probability of finding an electron pair.

An electron pair domain extends around the most probable position of finding an electron pair as determined by the Pauli principle and the probability decreases on increasing the distance from the most probable position.

A lone pair domain can in general be thought of as more spread out and larger than a bonding pair domain as the electrons of a lone pair are subject to the attraction of only one atomic nucleus while those of a bonding air are subject to the attraction of two nuclei taking part in the bond formation.

A bonding pair domain therefore taken up less space around a nucleus compared to a lone pair domain because the former is stretched out more towards the ligand/ substituent of the central atom. Why a lone pair occupies more space than a bonding pair is well explained by the domain concept.


Ligand close packing model

The basis of this model is that the bond angles are determined by the packing of the ligands or substituents around a central atom. Irrespective of having lone pairs or not, the distance maintained between the substituents on a central atom will be almost same while the bond lengths and bond angles will be different as shown in the following examples.


Platonic Solids

423 BC 347 BC

Archimedian Solids